Problems with Photoelectric Cell Theory

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Problems with Photoelectric Cell Theory

Postby Pivot on October 14th, 2019, 6:34 am 

A photo-voltaic cell is essentially a P-N junction with the n-type wafer exposed to light.

Referring to the diagram below:
Image

Electrons are generated from silicon atoms in the n-type and depletion zones by the photoelectric effect. The increase in free electrons numbers causes electrons to migrate towards the front electrical contact by hole -hopping (the term hole-hop is used to avoid the major/minor carrier terminology), and then to move down the connected wire towards the back contact.

Electrons arrive at the back electrical contact via the wire and start hole-hopping up through the p-type zone, so avoiding the electrical repulsion by the p-type anions. Sufficient electrons hole-hop to reach the depletion zone and thus to complete the circuit and maintain the continuous flow of electrons (i.e.the electric current).

The problem that I see here is that the electrons hole-hopping their way through the p-type layer are moving in the reverse bias direction for a P-N junction (see diagram below), and thus should not be able to pass into the depletion zone to complete the circuit. Can anyone resolve this dilemma?

http://www.sciencechatforum.com/download/file.php?id=6880
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Re: Problems with Photoelectric Cell Theory

Postby Pivot on October 14th, 2019, 11:32 pm 

Apart from accidentally uploading my question multiple times, it looks as if my diagrams have disappeared, so here they are again, this time on my Google drive.

https://drive.google.com/open?id=1tDZNV2-QpQkc_DkXJkzC4b8Eg-5XhXtX
and
https://drive.google.com/open?id=1pRxwk89Nyogo-EcZEA_rHZOXZHov_Pra

You may find it easier to just download the question complete with diagrams as the pdf.

https://drive.google.com/open?id=1JU5D84MVddY9hpqOlM8AfQbmNLSI37s-
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Re: Problems with Photoelectric Cell Theory

Postby bangstrom on October 15th, 2019, 1:14 am 

I couldn't get the link you gave to work. I see some possible problems with your description. The electrons are generated from the N-material used to dope the silicon and not from the silicon. The silicon serves as a medium for hole hopping and the positively charged holes are attractive to the negatively charged electrons. They don't repel. I hope that helps.
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Re: Problems with Photoelectric Cell Theory

Postby bangstrom on October 15th, 2019, 1:40 am 

I see your diagrams now but they look like two different things. The top diagram looks like a photocell where the energy is provided by the sun and the bottom diagrams appear to show a single diode where the current is supplied by a battery, Note that the polarity of the battery is the opposite in the two pictures. The diagram shows how a diode allows the current to flow in one direction but not the other,
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Re: Problems with Photoelectric Cell Theory

Postby Pivot on October 15th, 2019, 2:08 am 

The electrons in the n-type layer come from the dopant for a P-N junction. However in a photo-voltaic cell context a photon energy of 1.1 electron volts (infrared light) or greater has enough energy to interact with a bond in the silicon crystal and release more electrons, so creating hole. That is the beauty of the photoelectric effect: more electrons and an electricity source.

The repulsion mentioned was between p-type anions (negative) and the electrons. By hole hopping through the p-type silicon holes formed by the creation of those anions are the way electrons avoid the anion repulsion and trickle through the p-type towards the depletion zone, but that means they are moving in the reverse bias direction. Thus they should not be able to move across the depletion zone. If they cannot return this would represent a violation of the conservation of energy principle.

The single diode is a P-N junction. In the reverse bias diagram the electrons (on the negative terminal side) are attempting to move through the p-type layer, but cannot because the diode is turned OFF by electricity with that polarity. This is the same direction and mode of transport shown for electron returning towards the p-type layer in the photo-cell. That is the problem as I see it.
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Re: Problems with Photoelectric Cell Theory

Postby bangstrom on October 15th, 2019, 6:51 am 

If I am reading the illustration correctly, the electrons are moving upward to the negative wire while the holes move downward to through the P-type so the electrons don’t move through the P-type and they are moving in the forward bias direction.
Are you wondering why the electron current is moving in one direction (the wrong way) while the electrical current flows the other way?
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Re: Problems with Photoelectric Cell Theory

Postby Pivot on October 15th, 2019, 10:17 am 

Yes, the electrons move upwards in the n-type, into the wire, to load into a hole in the p-type. Electrons in the p-type (not necessarily the ones arriving via the wire) work their way up the p-type: as on jumps out of a silicon atom into an adjacent hole, its exit creates a new hole.

The net effect is that the holes appear to move down towards the back contact to meet and greet new electron arrivals (holes are such party animals) and for there to be a corresponding electron migration up towards the depletion zone. Thus the electrons (the electric current) percolate up the p-type as the holes percolate down. The direction of the electrons in the p-type is in the reverse bias direction and thus cannot get past the depletion zone.
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Re: Problems with Photoelectric Cell Theory

Postby bangstrom on October 15th, 2019, 1:27 pm 

Pivot » October 15th, 2019, 9:17 am wrote:
The net effect is that the holes appear to move down towards the back contact to meet and greet new electron arrivals (holes are such party animals) and for there to be a corresponding electron migration up towards the depletion zone. Thus the electrons (the electric current) percolate up the p-type as the holes percolate down. The direction of the electrons in the p-type is in the reverse bias direction and thus cannot get past the depletion zone.

As I see it, the electrons exit at the negative pole of the photocell and travel to the switch and then continue on to the back to the base of the photocell -the positive pole. Here they fall into the holes of the P-type and the holes become neutral silicon atoms again That is the completion of the circuit. There are no electrons moving up through the the P-type to return to the depletion zone. Does that make sense?
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Re: Problems with Photoelectric Cell Theory

Postby Pivot on October 15th, 2019, 11:06 pm 

None at all. That would mean that the holes would soon fill along the bottom contact and new arrival electrons would have nowhere to park their butts and the electron flow would stop. With the hole concept new holes to accommodate new arrivals can only be created by an electron jumping out of orbit, and the only way to go with the pressure of new arrivals is away from the contact.

Holes moving down implies electrons moving up, albeit by filling a higher hole.

It makes more sense that the electrons in the n-type move down and through the depletion zone and then the p-type in the forward bias direction. This would mean that the holes would be moving upwards through the p-type as the electrons move down. Thus the electrons and holes would be moving in the opposite direction to that indicated by this diagram and many like it.
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Re: Problems with Photoelectric Cell Theory

Postby Pivot on October 16th, 2019, 1:31 am 

Also, as an afterthought, if you look at how photoelectric cells are attached to a regulator, the isolation switch is at the positive terminal and the negative is to the common earth in the expectation that electricity travels from a positive to the negative terminal.

Thus they would connect the p-type side (the nominal positive terminal) of the cell to the active side of the circuit. A mis-labeling of the terminals on a solar cell would mean that the positive-to-positive connection between cell and regulator would mean the active end (i.e. from where the electrons will flow) is the positive end, justifying the location of the isolation switch.

If it is not a mis-labeling then the n-type side should be connected to the active side of the regulator as a negative-to-positive connection. But it isn't.
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Re: Problems with Photoelectric Cell Theory

Postby bangstrom on October 16th, 2019, 2:36 am 

Pivot » October 16th, 2019, 12:31 am wrote:

If it is not a mis-labeling then the n-type side should be connected to the active side of the regulator as a negative-to-positive connection. But it isn't.


You are right the diagram, and nearly all diagrams, are mislabeled thanks to Ben Franklin. Old Ben thought lightning flowed from the clouds to the ground and the visible part does so he called the electrical charge of lightning "positive." Ever since Franklin the current in circuit diagrams has been labeled as flowing from positive to negative.

When the electron was discovered many years later, it was found to have a negative charge so all the diagrams and books had the current flowing in the wrong direction. Rather than change the established convention, the circuit diagrams remain mislabeled while we know the electrons that make up the electron circuit are flowing from negative to positive. It has been terribly confusing ever since.

In the photocell diagram, light energy knocks electrons from the depletion zone of the P-type lattice and the electrons flow upward leaving the newly created holes behind. The free electrons and holes both come from the depletion zone where the p-type crystal is now “depleted” of electrons. If the electrons and holes were to recombine at this point, that would be the end of the current flow before it started. The electrons at this point have too much energy to drop back into a convenient hole so they leave through a conductor at the top of the solar cell.

The electrons pass through the circuit where they lose some of their energy and return to the bottom of the photocell attracted by the positively charged holes as they accumulate at the bottom. This is where the electrons and holes recombine as long as light energy continues to create more new free electrons at the top of the cell.
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Re: Problems with Photoelectric Cell Theory

Postby Pivot on October 16th, 2019, 11:13 pm 

Yes, I think we are getting closer to agreeing.
In the photocell diagram, light energy knocks electrons from the depletion zone of the P-type lattice and the electrons flow upward leaving the newly created holes behind.

The diagram is not correct in this regard. The photoelectric effect would diminish with depth, with more electrons being released closer to the incident surface , and progressively reducing in the n-type and depletion zones. Possibly it just makes the diagram too complicated and messy to add such detail.

Thus there are higher concentrations of electrons closer to the incident surface that are pushed by combined negative charge of the electrons lower down in the n-type to move into the wire and so form the electric current. For those electrons in the depletion zone it is more likely the negative charge of the p-type dopant anions that pushes them into the n-type.

As the electrons arrive at the back contact they then percolate up through the p-type to complete the circuit as holes correspondingly percolate downwards towards the back contact.

It would seem that if this scenario is correct then when a power source supplies the flow of electrons and is attached to a P-N junction in a reverse bias direction electrons cannot travel from the p-type to n-type layers. However when the electrons are generated within a P-N junction by the photons the electrons do travel from the p-type to n-type, which was the subject of my post.

If this is the case, then it is a wonder that some smart Physicist or electronics guru hasn't attached a capacitor and coil to a photo-voltaic cell to create a solar cell that can output AC current without the need for an inverter. Sounds like a money-maker for someone.
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Re: Problems with Photoelectric Cell Theory

Postby bangstrom on October 17th, 2019, 4:08 am 

Pivot » October 16th, 2019, 10:13 pm wrote:Thus there are higher concentrations of electrons closer to the incident surface that are pushed by combined negative charge of the electrons lower down in the n-type to move into the wire and so form the electric current. For those electrons in the depletion zone it is more likely the negative charge of the p-type dopant anions that pushes them into the n-type.


Yes, the greatest number of electrons should be found at the incident surface and the N-type crystal lattice is the only one capable of conducting electrons because it allows electrons to move freely through the medium.

Pivot » October 16th, 2019, 10:13 pm wrote:As the electrons arrive at the back contact they then percolate up through the p-type to complete the circuit as holes correspondingly percolate downwards towards the back contact.

The electrons drop back into the holes at the base to complete the circuit so there is no percolating upward.

Pivot » October 16th, 2019, 10:13 pm wrote:It would seem that if this scenario is correct then when a power source supplies the flow of electrons and is attached to a P-N junction in a reverse bias direction electrons cannot travel from the p-type to n-type layers. However when the electrons are generated within a P-N junction by the photons the electrons do travel from the p-type to n-type, which was the subject of my post.


Yes, the electrons travel a short distance from the p-type to the n-type. Some electrons from the n-type above fill the holes at the interface “depletion zone” saturating the zone with electrons. The loss of electrons from the n-layer leaves the area positive and allows the electrons to move upward.

Pivot » October 16th, 2019, 10:13 pm wrote:If this is the case, then it is a wonder that some smart Physicist or electronics guru hasn't attached a capacitor and coil to a photo-voltaic cell to create a solar cell that can output AC current without the need for an inverter. Sounds like a money-maker for someone.

Great idea but such a device has been invented and it is called an inverter. An inverter contains some additional solid state devices needed to convert the AC to the desired usable form.
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Re: Problems with Photoelectric Cell Theory

Postby Pivot on October 17th, 2019, 10:48 am 

There would seem to be only one area that we now disagree on regards this topic. You said
The electrons drop back into the holes at the base to complete the circuit so there is no percolating upward.


Sure there are holes already in the p-type from the dopant ionisation, but to accommodate the flood of electrons arriving at the back contact via the wire many more need to be made available. Thus more holes need to percolate down: a holey pilgrimage you might say. Holes moving down towards the back contact means electrons moving up in the opposite direction. So electrons must move up otherwise all the holes in the p-type (dopants are about 1%? mean limited the holes in the p-type to start with) would soon be exhausted and the electron flow would stop. When holes come down electrons go up.
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Re: Problems with Photoelectric Cell Theory

Postby bangstrom on October 17th, 2019, 1:26 pm 

More light at the top releases more electrons and one hole is created for every electron lost. There is a continuous flow of holes from the top to the bottom. I don't know how far the electrons can penetrate the p-type crystal, if any, but holes keep coming as long as the light shall last.
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Re: Problems with Photoelectric Cell Theory

Postby Pivot on October 17th, 2019, 9:33 pm 

Okay, then explain how holes migrate down without electrons migrating upwards. Keep in mind a hole is an electron deficiency in an atom fixed in a crystal structure. To move it... (please finish the sentence)
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Re: Problems with Photoelectric Cell Theory

Postby bangstrom on October 18th, 2019, 3:50 am 

Pivot » October 17th, 2019, 8:33 pm wrote:Okay, then explain how holes migrate down without electrons migrating upwards. Keep in mind a hole is an electron deficiency in an atom fixed in a crystal structure. To move it... (please finish the sentence)


To move an electron from a silicon atom below drops into a hole and the hole is then one atom closer to the bottom and so on all the way down. When the hole reaches the bottom, it is filled by one of the return electrons from the circuit.

I see what you mean now but I never thought of the electrons in the matrix as the electrons in motion since their motion is so slow.
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Re: Problems with Photoelectric Cell Theory

Postby Pivot on October 18th, 2019, 5:03 am 

Thank god we finally got there. And yes, the progress of the electrons in the p-type is slower than those down the wire simply on a cross-sectional area basis.

And now we have a couple of new Physics nerd jokes:

When's a diode not a diode. Answer: when it tries to get a sun tan

and

Hey diode, come out of the closet and into the sunshine: we know that you're bi.

adios
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